Deepwater accumulators provide a supply of pressurized working fluid for the control and operation of subsea equipment, such as through hydraulic actuators and motors. Typical subsea equipment may include, but is not limited to, blowout preventers (BOPS) that shut off the well bore to secure an oil or gas well from accidental discharges to the environment, gate valves for the control of flow of oil or gas to the surface or to other subsea locations, or hydraulically actuated connectors and similar devices.
Accumulators are typically divided pressure vessels with a gas section and a hydraulic fluid section that operate on a common principle. The principle is to precharge the gas section with an inert, dry, ideal gas (usually nitrogen or helium), pressurized to a pressure at or slightly below the anticipated minimum pressure required to operate the subsea equipment. Hydraulic fluid will then be added (or “charged”) to the accumulator in the separate hydraulic fluid section, increasing the pressure of the pressurized gas and the hydraulic fluid to the maximum operating pressure of the control system. The precharge pressure determines the pressure of the very last trickle of fluid from the fluid side of the accumulator, and the charge pressure determines the pressure of the very first trickle of fluid from the fluid side of the accumulator. The discharged fluid between the first and last trickle will be at some pressure between the charge and precharge pressure, depending on the speed and volume of the discharge and the ambient temperature during the discharge event. The hydraulic fluid introduced into the accumulator is therefore stored at the maximum control system operating pressure until the accumulator is discharged for the purpose of doing hydraulic work.
Accumulators generally come in three styles—the bladder type having a balloon type bladder to separate the gas from the fluid, the piston type having a piston sliding up and down a seal bore to separate the fluid from the gas, and the float type with a float providing a partial separation of the fluid from the gas and for closing a valve when the float approaches the bottom to prevent the escape of the precharging gas. A fourth type of accumulator is pressure compensated for water depth and adds the precharge pressure plus the ambient seawater pressure to the working fluid.
The precharge gas can be said to act as a spring that is compressed when the gas section is at its lowest volume/greatest pressure and released when the gas section is at its greatest volume/lowest pressure. Accumulators are typically precharged on the surface in the absence of hydrostatic pressure and subsequently charged with hydraulic fluid on the seabed under full hydrostatic pressure. The surface precharge pressure is limited by the pressure containment and structural design limits of the accumulator vessel under surface ambient conditions. Yet, as accumulators are used in deeper water, the efficiency of conventional accumulators decreases as application of hydrostatic pressure causes the gas to compress, leaving a progressively smaller volume of gas to charge the hydraulic fluid. The gas section must consequently be designed such that the gas still provides enough power to operate the subsea equipment under hydrostatic pressure even as the hydraulic fluid approaches discharge and the gas section is at its greatest volume/lowest pressure.
The use of accumulators at extreme water depths requires large aggregate accumulator volumes that increase the size and weight of the overall subsea equipment assemblies. Yet, offshore rigs continue moving further and further offshore to drill in deeper and deeper water. Because of the ever increasing envelop of operation, traditional accumulators are becoming unmanageable with regards to quantity and location inside existing stack frames. In some instances, it has even been suggested that in order to accommodate the increasing demands of the conventional accumulator system, a separate subsea skid may have to be run in conjunction with the subsea BOP stack in order to provide the required volume necessary at the limits of the water depth capability of the subsea BOP stack. With rig operators increasingly putting a premium on minimizing size and weight of the drilling equipment to reduce drilling costs, the size and weight of all drilling equipment must be optimized.
The bulk transmission of hydraulic power to accumulators are affected by the ambient pressure at the sea floor and requires their designs to account for: 1) hydrostatic effects; 2) consequential design pressure ratings for subsea hydraulic accumulator pre-charge; and 3) volume requirements to meet performance requirements for the external hydraulic actuator. Also, transmission of hydraulic power through pipes is subject to line pressure losses due to line geometry, length, and fluid conditions. Further, different external hydraulic actuators require differing regulated pressure, requiring the use of a plurality of regulators for each type of hydraulic actuator.
Prior approaches to addressing operation of subsea linear actuators have involved replacement of the linear hydraulic actuator with a rotary electric motor, transmission, clutch, and lock. However, electromechanical losses associated with the electric rotary motor and mechanical losses associated with the transmission, clutch, and lock have led to power inefficiencies and significant complexity increases that reduce reliability, availability, and maintainability of all electric solutions over all hydraulic solutions.